1. Field of the Invention
The present invention relates to active pixel sensors. In particular, the present invention relates to active pixel sensors manufactured under a complementary metal-oxide-silicon (CMOS) process.
2. Discussion of the Related Art
Active pixel sensors (APS) are used in imaging applications, such as digital cameras. APS are described, for example, in the articles: (a) "256X256 CMOS Active Pixel Sensor Camera-on-a-Chip" by Nixon et al., IEEE International Solid-State Conference (ISSCC96), pp.178-179, and (b) "Current-Mediated, Current-Reset 768X512 Active Pixel Sensor Array" by R. D. McGrath et al., IEEE International Solid-State Conference (ISSCC96), pp.182-183.
These articles describe APS arrays and associated digital logic circuits which are integrated into integrated circuits. Logic circuits integrated with APS arrays are, for example, circuits for performing the timing and control functions of a "camera-on-a-chip". Typically, a conventional CMOS process is used to manufacture such an integrated circuit, since both the logic circuits and the APS array can be formed using CMOS transistors and diodes of such a conventional process. A typical APS 100, which is the building block of an APS array, is shown in FIG. 1.
As shown in FIG. 1, APS 100 includes transistors 101, 102 and 103, and a photodiode 104. An on-chip current source 105 allows the state of the APS to be read out. The drain and source terminals of transistor 101 are respectively coupled to a reference supply (V.sub.ref) 106 and a cathode (107) of photodiode 104, whose anode is coupled to a ground or fixed reference voltage (V.sub.SS). The source terminal of transistor 101 drives the gate terminal of transistor 102, whose drain and source terminals are coupled respectively to a power supply (V.sub.cc) 109 and drain terminal 108 of transistor 103. Reference supply (V.sub.ref) 106 can be, but need not be, power supply (V.sub.cc) 109. The source terminal of transistor 103 is coupled to current source 105. During operation, a high reset voltage is initially provided at transistor 101 to pull node 107 up to a dark reference voltage (V.sub.reset). If the active reset voltage is high enough to keep transistor 101 in the linear region, dark reference voltage V.sub.reset equals V.sub.ref. Keeping transistor 101 in the linear region is desired because the dark reference voltage V.sub.reset is then immune from noise in the threshold voltage (V.sub.T) of transistor 101. When the reset voltage is turned off, the charge trapped at photodiode 104's cathode (i.e., node 107) maintains a high voltage there. When APS 100 is exposed to light, photodiode 104 discharges node 107 to bring the voltage at node 107 towards the ground reference voltage. The voltage at node 107 can be read by turning on transistor 103, by applying a selection voltage at the gate terminal of transistor 103, and sensing the output voltage V.sub.out at terminal 120. For an undischarged pixel, voltage V.sub.out is given by: EQU V.sub.out =V.sub.reset -V.sub.noise -V.sub.T
where V.sub.reset is the dark reference voltage at node 107, V.sub.noise represents a reset noise, and V.sub.T is the threshold voltage for transistor 102. Because of the functions they perform, transistors 101 and 102 are often referred to as a "reset switch" and a "read-out amplifier," respectively.
As discussed above, if transistor 101 is a typical CMOS transistor, transistor 101 can operate in the linear region, so that V.sub.reset can be made very close to reference supply voltage V.sub.ref. In a transistor typically used in a CMOS logic circuit, the threshold voltage V.sub.T is approximately 0.8 volts. Such a threshold voltage is typically set by a P-type "V.sub.t adjustment" implant into the channel region. With a 3.3 volts back-bias (i.e., a source terminal voltage of 3.3 volts relative to the substrate), V.sub.T can be in excess of 1 volt. Consequently, V.sub.out has an output swing of less than 2 volts between the undischarged state and the discharged state of APS 100, as shown in the oscilloscope trace in FIG. 2. If the reset voltage at the gate terminal of transistor 101 is set to V.sub.ref, V.sub.reset is approximately V.sub.ref -V.sub.T, the output swing is even less. Thus, the active pixel sensor of the prior art has poor performance under low power supply conditions.